The Influence of Immune Complex-Bearing Follicular Dendritic Cells on the IgM Response, Ig Class Switching, and Production of High Affinity IgG

Abstract

It is believed that Ag in immune complexes (ICs) on follicular dendritic cells (FDCs) selects high affinity B cells and promotes affinity maturation. However, selection has been documented in the absence of readily detectable ICs on FDCs, suggesting that FDC-ICs may not be important. These results prompted experiments to test the hypothesis that IC-bearing murine FDCs can promote high affinity IgG responses by selecting B cells after stimulating naive IgM+ cells to mature and class switch. Coculturing naive λ+ B cells, FDCs, (4-hydroxy-3-nitrophenyl)acetyl-chicken γ-globulin (CGG) + anti-CGG ICs, and CGG-primed T cells resulted in FDC-lymphocyte clusters and production of anti-4-hydroxy-5-iodo-3-nitrophenyl acetyl. Class switching was indicated by a shift from IgM to IgG, and affinity maturation was indicated by a change from mostly low affinity IgM and IgG in the first week to virtually all high affinity IgG anti-4-hydroxy-5-iodo-3-nitrophenyl acetyl in the second week. Class switching and affinity maturation were easily detectable in the presence of FDCs bearing appropriate ICs, but not in the absence of FDCs. Free Ag plus FDCs resulted in low affinity IgG, but affinity maturation was only apparent when FDCs bore ICs. Class switching is activation-induced cytidine deaminase (AID) dependent, and blocking FDC-CD21 ligand-B cell CD21 interactions inhibited FDC-IC-mediated enhancement of AID production and the IgG response. In short, these data support the concept that ICs on FDCs can promote AID production, class switching, and maturation of naive IgM+ B cells, and further suggest that the IC-bearing FDCs help select high affinity B cells that produce high affinity IgG.

Establishment of Lymphotoxin β Receptor Signaling-Dependent Cell Lines with Follicular Dendritic Cell Phenotypes from Mouse Lymph Nodes

“…Primary FDCs prepared from lymphoid organs in irradiated mice have been used in many in vitro experiments reported to date (25)(26)(27)34). B cell viability, proliferation, and Ab production have been reported to be markedly enhanced when B cells were cocultured with the FDC preparations at high FDC:B cell ratio, such as 1:2ϳ1:5, which may be, however, too high compared with the ratio in GCs (1:100 or lower) (29).…”

“…Although these cell lines have been shown to retain a variety of FDC functions, some respects of them, including expression of surface markers, were different from those of primary FDCs. In previous in vitro experiments using primary FDCs that were isolated from murine lymphoid tissues, high FDC:B cell ratios, such as 1:3, have been used to observe augmenting effects of FDCs on the differentiation of B cells (25)(26)(27)34). These experimental conditions might not reproduce in vivo situations accurately because it has been pointed out that the ratio may be much lower (1:100 or lower) in GCs (29).…”

“…It has been widely believed that the criterion for the positive selection of GC B cells is the binding avidity to immune complexes (ICs) that are trapped on the surface of FDCs (25)(26)(27). However, controversies have been raised recently as to whether ICs retained on FDCs are essential as a ligand for the positive selection of GC B cells, because affinity maturation still occurred in the mutant mice that do not produce soluble Abs, and thus deposit no significant ICs on FDCs (28,29).…”

“…to trap the Abs onto the solid phase (27,41). IgM and IgG Abs bound to the plates were assayed with HRP-conjugated goat IgG Abs to each class.…”

Complement Activation and Complement Receptors on Follicular Dendritic Cells Are Critical for the Function of a Targeted Adjuvant

“…The follicular DCs (FDC) play a central role in the GC reaction; they retain Ag in the form of immune complexes (IC) within the GC via complement receptors or Fc receptors FcgIIB (CD32) and FcεRII (CD23) (21,22). Ags presented as IC on FDC have been shown to be highly stimulatory and provide a depot of Ag, a quality thought to be important for selecting high-affinity B cell clones in the GC, although this concept still remains somewhat controversial (23)(24)(25)(26). FDC are also a source of several modulating and survival signals that help maintain the GC, including CXCL13, BAFF, and IL-6, which are secreted by the FDC upon activation (27,28).…”